I wouldn't say that "Ένα βιβλίο καλό" is false. We sometimes do place the adjective after the noun; it's just less common.

Quoting from this webpage:Η θέση του επιθέτου ποικίλλει. Συνήθως τοποθετείται πριν από το ουσιαστικό, αλλά μπορεί να τοποθετείται και μετά από αυτό.(= The position of the adjective varies. It is usually placed before the noun, but it can also be placed after it.)

I also wanted to say that adjectives following the noun are definitely acceptable, but they have different connotations. At a first level, saying "ένα βιβλίο καλό" emphasizes "καλό", either as new information, or as the important part of the sentence. And at a second level, it creates the expectation that more descriptions will follow, eg "ένα βιβλίο καλό και φτηνό" is what my brain auto-completes. "ένα Χ καλό και φτηνό" is a common co-location of adjectives.

"If you like your clause structure, you can keep your clause structure"